The present invention relates to a process for preparing catalysts, which comprises depositing an active component or a compound to be converted to the active component ("precursor") on a supporting material by means of deposition-precipitation.
In solid catalysts the area of the active surface per unit volume of the catalyst and the accessibility of this surface to reactants are decisive of the activity and selectivity. In general, catalysts to be technically used must have a good stability. This means that the active material may not sinter at the temperatures at which the thermal pretreatment and the catalytic reaction take place. Because substantially all the catalytically active materials sinter rapidly at elevated temperatures, the active material is often applied to a so-called support.
The support, which is generally not catalytically active itself, is thermostable and therefore does not sinter at high temperatures. The specific area and the porous structure of the support are adapted to the catalytic reaction to be conducted, if possible. As far as catalytic reactions are concerned in which especially the activity at low temperature is important, the largest possible active surface area per unit volume of the catalyst is aimed at. In those cases the process is carried out with a highly porous support, the active material is finely and homogeneously distributed over the surface of the support with a high degree of loading. There are cases in which minute active particles show a relatively low activity per unit area. In those situations somewhat larger particles will preferably be applied to the surface of the support. This also applies if the selectivity of minute particles is unfavourable. In other catalytic reactions in which the selectivity is of decisive significance, wide-pored supports having a smaller specific surface area have proved to be superior. In all cases, however, a uniform, compact distribution of the active particles over the surface of the support is highly favourable.
There is a marked need for a process which renders it possible to apply catalytically active particles or precursors thereof having controllable sizes to the surface of supports in a uniformly distributed and highly compact state. To this end, the method of deposition-precipitation giving excellent results was developed previously. For extensive descriptions of this method reference is made to: A. J. van Dillen, J. W. Geus, L. A. M. Hermans and J. van der Meijden in Proceedings Sixth International Conference and Catalysis London (1976) and L. A. M. Hermans and J. W. Geus in Preparation of Catalysts II (B. Delmon, P. Grange, P. Jacobs and G. Poncelet, editors), pages 113-130, Elsevier, Amsterdam (1979). In deposition-precipitation the concentration of an insoluble (precursor of a) catalytically active component in a suspension of the support is homogeneously increased. If nucleation of the insoluble (precursor of the) catalytically active component preferentially proceeds on the surface of the suspended support, the homogeneous increase of the concentration leads to precipitation of (the precursor of) the active material on the support only. By controlling the amount of the (precursor of the) active component to be precipitated per unit area of the support, the sizes of the particles deposited on the support can be controlled.
A condition for obtaining deposition-precipitation is a sufficiently strong interaction of the material to be precipitated with the surface of the support; this is necessary for effecting preferential nucleation on the surface of the support. In order to establish whether the precipitation proceeds only on the support, the variation of the concentration of the compounds to be precipitated can be monitored during precipitation from homogeneous solution with and without a suspended support. If the concentration is lower during (the beginning of) the precipitation in the presence of a suspended support, the interaction with the surface of the support is sufficient to lead to selective nucleation on the surface of the support.
In a large number of cases the desired deposition-precipitation can be realized by homogeneously increasing the hydroxyl ion concentration of a suspension of the support. The pH value can be readily increased, also on a technical scale, e.g., by hydrolyzing urea or isocyanate. This is also possible by injecting a solution of alkaline reaction below the surface of a suspension of a support at an appropriate velocity.
A number of catalytically important elements or compounds, however, cannot be precipitated on to the support in the desired manner by increasing the pH value of a suspension of the support in a solution of the catalytically active precursor. This particularly applies to metal ions the stable form of which has such a high charge that they occur as oxy anions. Examples are vanadates, molybdates, tungstates and antimonates. By increasing the pH value of a solution of these ions, these anions, of course, do not precipitate. These anions can be precipitated by pH reduction; the pH can be homogeneously reduced by injection of an acid solution below the surface of a suspension of the support. In a number of important cases, however, there is no sufficient interaction with the surface of the suspended support. This poor interaction especially occurs with silica and silica/alumina supports.
There is therefore a need for a process which renders it possible to precipitate metal ions occurring in the stable form as anions on to/the support by increasing the pH value of a suspension of the support.